1. Noun

   1.

   main constituent; core; nucleus

2. 2.

   subject

3. Wikipedia definition

   3.

   Subject

Example sentences

• 社長が社員の主体的な取り組みを促した。

  The president urged employees to act on their initiative.

• 主体・客体さえ訳せればあとはそこまで難しくないです。

  If you can translate the subject and object then after that it isn't so difficult.